The radiation, sound, or noise emitted by an electronic product has been increasingly important. It may be desirable to limit such noise to avoid interference with the electronics of nearby objects or vehicles. Governments may also set stringent standards for such radiation emissions. Accordingly, a large number of consumer, industrial, and military products and industrial equipment must now undergo radiation emission testing.
Anechoic chambers are frequently employed in such radiation emissions tests. Anechoic chambers are rooms specifically designed to completely absorb sound or electromagnetic waves. The interior walls, floor, and ceilings of these chambers are lined with sound (acoustically absorbent) or electromagnetic absorbing (radiation absorbent) materials to eliminate interior reflected sound. The exteriors of these chambers are also insulated to allow predictable transmission loss characteristics from the exterior of the anechoic chamber to the interior of the anechoic chamber. These chambers reduce sound or electromagnetic reflections and external noises to test acoustics (sound waves), antennae, radars, of the electromagnetic interference of other electronics. All sound or electromagnetic energy emitted from the object being tested will be traveling away from the test object with almost none reflected back.
Many of these anechoic chambers can be quite large, since testing can be conducted on full-scale objects, including aircraft or other vehicles.
The radiation absorbent materials of these chambers are designed and shaped to absorb radio frequency radiation from as many directions as possible. One of the most effective types of radiation absorbent structures comprises arrays of pyramid shaped pieces, or cones composed of radiation absorbent materials. Usually these cones will comprise a foam material containing mixtures of carbon and iron. The length from base to tip of the pyramid structure is chosen based on the lowest expected frequency and the amount of absorption required. For low frequency damping, this distance is often 24 inches, while high frequency panels are as short as 3-4 inches.
To work effectively, all internal surfaces (walls, ceiling, and floor) of the anechoic chamber must be completely covered with these cones. These cones are installed with the tips pointing inward to the chamber. Sections of these cones may be temporarily removed to install equipment but they must be replaced before performing any tests. Any testing equipment (as opposed to the equipment under test) to be used within the anechoic chambers must expose as few metallic surfaces as possible, as these risk causing unwanted radiation reflections.
A significant problem with anechoic chambers is that these radiation absorbent structures are very flammable. As the radiation absorbent cones absorb radiation, this absorption generates heat within the cones. If this generated heat cannot be dissipated there is a risk that hot spots may develop and the temperatures within the cones may rise to the point of combustion. Testing at high frequencies can start the cone material on fire. Once this fire begins, it will spread rapidly through the remaining flammable cones.
Unfortunately, standard fire suppression systems, such as water sprinklers, cannot be installed within anechoic chambers since these suppression systems are constructed of materials or in configurations that would interfere with the radiation absorbent structures. Some radiation absorbent structures can be treated with fire retardants to reduce the risks of combustion, but these retardants cannot completely eliminate the risks of fire.